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Message-Id: <45CD97A1-F3CF-4B26-9DEC-3A2694A1FFF6@collabora.com>
Date: Tue, 25 Nov 2025 13:12:27 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Onur Özkan <work@...rozkan.dev>
Cc: rust-for-linux@...r.kernel.org,
lossin@...nel.org,
lyude@...hat.com,
ojeda@...nel.org,
alex.gaynor@...il.com,
boqun.feng@...il.com,
gary@...yguo.net,
a.hindborg@...nel.org,
aliceryhl@...gle.com,
tmgross@...ch.edu,
dakr@...nel.org,
peterz@...radead.org,
mingo@...hat.com,
will@...nel.org,
longman@...hat.com,
felipe_life@...e.com,
daniel@...lak.dev,
bjorn3_gh@...tonmail.com,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 2/6] rust: implement `Class` for ww_class support
> On 1 Nov 2025, at 13:10, Onur Özkan <work@...rozkan.dev> wrote:
>
> Adds the `Class` type, the first step in supporting
> `ww_mutex` in Rust. `Class` represents ww_class, used
> for deadlock avoidance for supporting both wait-die
> and wound-wait semantics.
>
> Also adds the `define_class!` macro for safely declaring
> static instances.
>
> Signed-off-by: Onur Özkan <work@...rozkan.dev>
> ---
> rust/kernel/sync/lock.rs | 1 +
> rust/kernel/sync/lock/ww_mutex.rs | 7 ++
> rust/kernel/sync/lock/ww_mutex/class.rs | 156 ++++++++++++++++++++++++
> 3 files changed, 164 insertions(+)
> create mode 100644 rust/kernel/sync/lock/ww_mutex.rs
> create mode 100644 rust/kernel/sync/lock/ww_mutex/class.rs
>
> diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
> index 27202beef90c..5b320c2b28c1 100644
> --- a/rust/kernel/sync/lock.rs
> +++ b/rust/kernel/sync/lock.rs
> @@ -15,6 +15,7 @@
>
> pub mod mutex;
> pub mod spinlock;
> +pub mod ww_mutex;
>
> pub(super) mod global;
> pub use global::{GlobalGuard, GlobalLock, GlobalLockBackend, GlobalLockedBy};
> diff --git a/rust/kernel/sync/lock/ww_mutex.rs b/rust/kernel/sync/lock/ww_mutex.rs
> new file mode 100644
> index 000000000000..727c51cc73af
> --- /dev/null
> +++ b/rust/kernel/sync/lock/ww_mutex.rs
> @@ -0,0 +1,7 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Rust abstractions for the kernel's wound-wait locking primitives.
> +
> +pub use class::Class;
> +
> +mod class;
> diff --git a/rust/kernel/sync/lock/ww_mutex/class.rs b/rust/kernel/sync/lock/ww_mutex/class.rs
> new file mode 100644
> index 000000000000..d10db5da01d2
> --- /dev/null
> +++ b/rust/kernel/sync/lock/ww_mutex/class.rs
> @@ -0,0 +1,156 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Provides [`Class`] to group wound/wait mutexes to be acquired together
> +//! and specifies which deadlock avoidance algorithm to use (e.g., wound-wait
> +//! or wait-die).
> +//!
> +//! The [`define_class`] macro and [`Class::new_wait_die`]/[`Class::new_wound_wait`]
> +//! constructors provide safe ways to create classes.
> +
> +use crate::bindings;
> +use crate::prelude::*;
> +use crate::types::Opaque;
> +
> +/// Creates static [`Class`] instances.
> +///
> +/// # Examples
> +///
> +/// ```
> +/// use kernel::{c_str, define_class};
> +///
> +/// define_class!(WOUND_WAIT_GLOBAL_CLASS, wound_wait, c_str!("wound_wait_global_class"));
> +/// define_class!(WAIT_DIE_GLOBAL_CLASS, wait_die, c_str!("wait_die_global_class"));
> +/// ```
> +#[macro_export]
> +macro_rules! define_class {
> + ($name:ident, wound_wait, $class_name:expr) => {
> + static $name: $crate::sync::lock::ww_mutex::Class =
> + // SAFETY: This is `static`, so address is fixed and won't move.
> + unsafe { $crate::sync::lock::ww_mutex::Class::unpinned_new($class_name, false) };
> + };
> + ($name:ident, wait_die, $class_name:expr) => {
> + static $name: $crate::sync::lock::ww_mutex::Class =
> + // SAFETY: This is `static`, so address is fixed and won't move.
> + unsafe { $crate::sync::lock::ww_mutex::Class::unpinned_new($class_name, true) };
> + };
> +}
> +
> +/// Used to group mutexes together for deadlock avoidance.
> +///
> +/// All mutexes that might be acquired together should use the same class.
> +///
> +/// # Examples
> +///
> +/// ```
> +/// use kernel::sync::lock::ww_mutex::Class;
> +/// use kernel::c_str;
> +/// use pin_init::stack_pin_init;
> +///
> +/// stack_pin_init!(let _wait_die_class = Class::new_wait_die(c_str!("some_class")));
> +/// stack_pin_init!(let _wound_wait_class = Class::new_wound_wait(c_str!("some_other_class")));
> +///
> +/// # Ok::<(), Error>(())
> +/// ```
> +#[pin_data]
> +#[repr(transparent)]
> +pub struct Class {
> + #[pin]
> + pub(super) inner: Opaque<bindings::ww_class>,
> +}
> +
> +// SAFETY: [`Class`] is set up once and never modified. It's fine to share it across threads.
> +unsafe impl Sync for Class {}
> +// SAFETY: Doesn't hold anything thread-specific. It's safe to send to other threads.
> +unsafe impl Send for Class {}
> +
> +impl Class {
> + /// Creates an unpinned [`Class`].
> + ///
> + /// # Safety
> + ///
> + /// Caller must guarantee that the returned value is not moved after creation.
> + pub const unsafe fn unpinned_new(name: &'static CStr, is_wait_die: bool) -> Self {
> + Class {
> + inner: Opaque::new(bindings::ww_class {
> + stamp: bindings::atomic_long_t { counter: 0 },
> + acquire_name: name.as_char_ptr(),
> + mutex_name: name.as_char_ptr(),
> + is_wait_die: is_wait_die as u32,
> + // TODO: Replace with `bindings::lock_class_key::default()` once
> + // stabilized for `const`.
> + //
> + // SAFETY: This is always zero-initialized when defined with
> + // `DEFINE_WD_CLASS` globally on C side.
> + //
> + // For reference, see __WW_CLASS_INITIALIZER() in
> + // "include/linux/ww_mutex.h".
> + acquire_key: unsafe { core::mem::zeroed() },
> + // TODO: Replace with `bindings::lock_class_key::default()` once
> + // stabilized for `const`.
> + //
> + // SAFETY: This is always zero-initialized when defined with
> + // `DEFINE_WD_CLASS` globally on C side.
> + //
> + // For reference, see __WW_CLASS_INITIALIZER() in
> + // "include/linux/ww_mutex.h".
> + mutex_key: unsafe { core::mem::zeroed() },
> + }),
> + }
> + }
> +
> + /// Creates a [`Class`].
> + ///
> + /// You should not use this function directly. Use the [`define_class!`]
> + /// macro or call [`Class::new_wait_die`] or [`Class::new_wound_wait`] instead.
> + fn new(name: &'static CStr, is_wait_die: bool) -> impl PinInit<Self> {
> + pin_init! {
> + Self {
> + inner <- Opaque::ffi_init(|slot: *mut bindings::ww_class| {
> + // SAFETY: The fields are being initialized. The `name` pointer is valid for a
> + // static lifetime. The keys are zeroed, which is what the C side does.
> + unsafe {
> + slot.write(bindings::ww_class {
> + stamp: bindings::atomic_long_t { counter: 0 },
> + acquire_name: name.as_char_ptr(),
> + mutex_name: name.as_char_ptr(),
> + is_wait_die: is_wait_die.into(),
> + // TODO: Replace with `bindings::lock_class_key::default()` once
> + // stabilized for `const`.
> + //
> + // SAFETY: This is always zero-initialized when defined with
> + // `DEFINE_WD_CLASS` globally on C side.
> + //
> + // For reference, see __WW_CLASS_INITIALIZER() in
> + // "include/linux/ww_mutex.h".
> + acquire_key: core::mem::zeroed(),
> + mutex_key: core::mem::zeroed(),
> + });
> + }
> + }),
> + }
> + }
> + }
> +
> + /// Creates wait-die [`Class`].
> + pub fn new_wait_die(name: &'static CStr) -> impl PinInit<Self> {
> + Self::new(name, true)
> + }
> +
> + /// Creates wound-wait [`Class`].
> + pub fn new_wound_wait(name: &'static CStr) -> impl PinInit<Self> {
> + Self::new(name, false)
> + }
> +
> + /// Creates a `Class` from a raw pointer.
> + ///
> + /// This function is intended for interoperability with C code.
> + ///
> + /// # Safety
> + ///
> + /// The caller must ensure that `ptr` points to the `inner` field of
> + /// [`Class`] and that it remains valid for the lifetime `'a`.
> + pub const unsafe fn from_raw<'a>(ptr: *mut bindings::ww_class) -> &'a Self {
> + // SAFETY: By the safety contract, `ptr` is valid to construct `Class`.
> + unsafe { &*ptr.cast() }
> + }
> +}
> --
> 2.51.2
>
>
Reviewed-by: Daniel Almeida <daniel.almeida@...labora.com>
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